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8.9.1 Suspension Insulators: Strength rating methods and nomenclature vary depending on the insulator material.

For porcelain, ANSI C29.1 specifies Mechanical and Electrical (M&E) procedures. The M&E value is determined by a combined mechanical and electrical test. The insulator has a voltage (75 percent of its rated dry flashover) impressed across it while a mechanical load is gradually applied to the insulator. For non-ceramics, most manufacturers conduct specified mechanical loading (SML) procedures to determine a polymer insulator’s failure rating. These procedures are similar to the M&E for porcelain, but no electrical test is applied.

ANSI C 29.2 defines standard mechanical ratings for porcelain as: 15,000 lbs., 25,000 lbs., 36,000 lbs. and 50,000 lbs. ANSI C29.12 defines standard SML’s for non-ceramic transmission insulators as: 20,000 lbs., 25,000 lbs., 36,000 lbs. and 40,000 lbs.

For recommended insulator loading limits, refer to Table 8-5. Under NESC district loading conditions, suspension insulators should not be loaded to more than 40 percent of their standard ANSI M&E rating for porcelain insulators or 40 percent of their ANSI SML for non-ceramics.

If a heavier loading than the NESC district loading can be expected to occur with reasonable regularity, then the 40 percent loading limit should be maintained at the higher loading limit.

Under extreme ice or high wind (50-year mean recurrence interval wind conditions) the load on the insulator should not exceed 65 percent of the M&E strength of the insulator for porcelain and 50 percent of the M&E strength for non-ceramics.

Generally, porcelain insulators with a 15,000 pound M&E rating will be satisfactory for tangent structures. However, stronger insulators may be needed on long spans with large conductors and at deadends and angles where the insulators carry the resultant conductor tension.

TABLE 8-5

SUMMARY OF RECOMMENDED INSULATOR LOADING LIMITS

Insulator Type NESC District Loading Extreme Loading

Non-ceramic Porcelain

Suspension 40% 50% 65%

(% of ANSI standard SML or M&E strength)

(% of ANSI standard SML strength)

(% of ANSI standard M&E strength)

Horizontal Post

Cantilever 40% 50% 65%

Tension, Compression 50%

(% of appropriate rated ultimate strength value)

50%

(% of appropriate rated ultimate strength value)

65%

(% of appropriate rated ultimate strength value)

Vertical Post (Porcelain) 750 lbs. ---

Vertical Pin Insulator (Porcelain, Mounted on the Crossarm)

500 lbs. ---

When suspension non-ceramic insulators are used, the designer must be aware of the effects on insulator swing calculations due to increased length and reduced weight. Agency Bulletin 1724E-220, “Procurement and Application Guide for Non-Ceramic Composite Insulators,”

provides additional information on non-ceramic insulators. When used as a jumper, polymer suspension insulators may be pulled towards the structure because of their lightweight.

8.9.2 Horizontal Post Insulators (Porcelain and Non-ceramic): Under NESC loading district conditions, horizontal post insulators must not be loaded to more than 40 percent of their

ultimate cantilever strength. As with suspension insulators, if a loading more severe than the NESC loading can be expected to occur with reasonable regularity, then the limit recommended for the more severe loading should be used. Under extreme ice conditions, the cantilever load on horizontal post insulators should not exceed 65 percent of the ultimate strength for porcelain and 50 percent of the ultimate strength for non-ceramics.

When a line angle is turned at a horizontal post structure, some or all of the insulators will be in tension. Under standard NESC loading conditions, the tension or compression load on the insulator must not exceed 50 percent of the ultimate tension or compression strength of the insulator. Under extreme loading conditions, the tension load on the insulator must not exceed 65 percent of the ultimate tension strength for porcelain and 50 percent of the ultimate tension strength of non-ceramic insulators.

Line post insulators are actually subjected to vertical, transverse and longitudinal loads

simultaneously. These loads represent the actual applied stresses to the line post insulator core that are experienced in the field. Vertical, transverse and longitudinal loads each contribute to the total bending moment, or total stress on the rod. Non-ceramic manufacturers provide combined loading application curves, which represent the mechanical strength limits of a non-ceramic line post insulator when subjected to simultaneous loads. These curves are used to determine how the insulator’s combined loading requirements compare with its cantilever (bending) strength. The combined loading application curves are used during the engineering stage to evaluate the mechanical strength of the insulator for specific line loading criteria.

There are three special considerations that must be mentioned in relation to horizontal post insulators:

Insulator Grounding: Where the structure ground wire passes near horizontal post insulators, it either should be stood off from the pole by means of a non-conducting strut or must be solidly bonded to the base of the insulator. This grounding is necessary to avoid radio noise problems.

Mechanical Impact Failures: Porcelain post insulators mounted on steel, concrete, or (in some cases) on wood structures using H-class poles, have experienced cascading mechanical failures due to impact loads because of the relative rigidity of the structures. To minimize the affects of impact loads, it is recommended that on rigid structures, non-ceramic insulators be used, or that porcelain post insulators be equipped with deformable bases, shear pin devices, or other means of relieving mechanical overloads.

Live Line Maintenance Issues: Many compact designs restrict the lineman for working on transmission lines while energized. Rule 441 of the NESC provides Table 441-1 which gives the recommended AC live work minimum approach distance for various voltages.

8.9.3 Porcelain Vertical Post and Pin Insulators Mounted on Crossarms: The maximum transverse load should be limited to 500 lbs. for standard single pin type agency standard structures and 750 lbs for standard vertical post type structures. The 500 lb. limit applies

whether the load is from standard NESC loading district loadings alone or from a combination of loading district loading

and the resultant of conductor tension on line angles. These limit will prevent excessive stress on the insulator, the tie wires (if used), insulator pin (if used), and the wood crossarm. The transverse load can be doubled by using double pin or post construction. See Table 8-5 for a summary of recommended insulator loading limits.

8.9.4 Coordination of Insulator Strength with Strength of Associated Hardware: Care should be taken to coordinate the strength of the hardware associated with the insulator with the strength of the insulator itself.

8.9.5 Example of Maximum Vertical Span Due to Horizontal Post Insulator Strength:

A 115 kV line is to be built using horizontal post insulators with a cantilever strength of

2,800 lbs. The conductor to be used is 477 kcmil 26/7 ACSR. Determine the maximum vertical span under:

1. Heavy loading district conditions; and

2. Under an extreme ice load, no wind, and 1.5 in. of radial ice

(See Chapter 11 for definitions of heavy loading and Chapter 9 for information on conductors).

Solution: From Appendix B, Conductors, the weights per unit length for the two conditions of the conductor are:

Heavy Loading District of 1/2 inch radial ice = 1.5014 lbs./ft.

Extreme radial ice of 1.5 inch =5.0554 lbs./ft.

Span Limits for Heavy Loading District:

2800 lbs.(0.40) = 746 ft.

1.5014 lbs./ft.

Span Limits for Extreme Ice Condition:

2800 lbs.(0.65) = 360 ft.

5.0554 lbs./ft.

The maximum vertical span is therefore 360 ft.

8.9.6 Example of Determining Minimum Suspension Insulator M&E Rating: A conductor has a maximum tension under heavy loading district conditions of 10,000 1bs. Under extreme radial ice of 1.5 in, it has a maximum tension of 16,000 lbs. Determine the minimum M&E rating of suspension bell insulators to be used in tension strings. (Tension strings are those insulator strings that are in line with the conductor and bear its full tension).

Solution:

Under NESC loading district conditions, the insulator can be loaded up to 40 percent of its M&E rating. Therefore:

(M&E rating)(0.4) = load M&E rating = load/(0.4)

M&E rating = 10000 lbs./(0.4) = 25000 lbs.

Under extreme ice conditions the insulator can be loaded to 50 percent of its M&E rating.

Therefore:

(M&E rating)(.65) = load M&E rating = load/(0.65)

M&E rating = 16,000 lbs./(0.65) = 24,615 lbs.

c. Based on ANSI standard M&E ratings, the insulators to be used should have a minimum standard rating of 25,000 lbs.